The present invention relates to optically absorptive photonic devices and in particular photovoltaic and photoconductive devices and their formation. Embodiments of the invention relate particularly to devices formed from multiple semiconducting layers, preferably composed of organic semiconducting polymers.
Semiconductive photovoltaic devices are based on the separation of electron-hole pairs formed following the absorption of a photon. An electric field is generally used for the separation. The electric field may arise from a Schottky contact where a built-in potential exists at a metal-semiconductor interface or from a pn junction between p-type and n-type semiconductive materials. Such devices are commonly made from inorganic semiconductors especially silicon which is used in monocrystalline, polycrystalline or amorphous forms. Silicon is normally chosen because of its high conversion efficiencies and the large industrial investments which have already been made in silicon technology. However, silicon technology has associated high costs and complex manufacturing process steps resulting in devices which are expensive in relation to the power they produce.
xe2x80x9cTwo-layer organic photovoltaic cellxe2x80x9d, Applied Physics Letters 48(2), Jan. 13, 1986, C. W. Tang, U.S. Pat. No. 4,164,431 and U.S. Pat. No. 4,281,053 describe multi-layer organic photovoltaic elements. These devices are formed in a layer by layer fashion. A first organic semiconductive layer is deposited on an electrode, a second organic semiconductive layer is deposited on the first organic layer and an electrode is deposited on the second organic layer. The first and second organic semiconductive layers are electron acceptors and hole acceptors. In the following, an xe2x80x9celectron accepting materialxe2x80x9d refers to a material which due to a higher electron affinity compared to another material is capable of accepting an electron from that material. A xe2x80x9chole accepting materialxe2x80x9d is a material which due to a smaller ionisation potential compared to another material is capable of accepting holes from that other material. The absorption of light in organic photoconductive materials results in the creation of bound electron-hole pairs, which need to be dissociated before charge collection can take place. The material considerations for organic devices are different compared to inorganic devices, where the electron and holes created by the absorption of a photon are only weakly bound. The dissociation of the bound electron-hole pair is facilitated by the interface between the layer of material which acts as a hole acceptor and the layer of semiconductive material which acts as an electron acceptor. The holes and electrons travel through their respective acceptor materials to be collected at the electrodes.
The designing of photovoltaic devices which are fabricated in a layer by layer fashion is limited. When one organic layer is deposited on top of another organic layer, the second layer must be added in such a way that the previously deposited layer is not affected in a detrimental way. Consequently solvents used for subsequent layers are limited in order not to dissolve the previous layer completely or destroy it in other ways. xe2x80x9cEfficient photodiodes from interpenetrating polymer networksxe2x80x9d, Nature, vol 376, Aug. 10, 1995, page 498-500, J. J. M. Halls et al, and U.S. Pat. No. 5,670,791 describe the formation of a photovoltaic device by depositing a single layer comprising a blend of first and second semiconductive polymers and the deposition of a second electrode on top of that layer. The first semiconductive polymer acts as a electron acceptor and the second semiconductive polymer acts as a hole acceptor. The first and second semiconductive polymers form respective continuous networks that interpenetrate so that there is a continuous path through each of the semiconductive polymers and a charge carrier within one of the first and second semiconductive polymers can travel between the first and second electrodes without having to cross into the other semiconductive polymer. However, these devices do not show the high efficiency that would be expected if the devices worked as ideally envisaged. This may be due to the fact that it is likely that at least one of the polymers can extend through the whole device, thereby creating a parallel system of single material diodes.
It is an object of the present invention to provide an improved photovoltaic device.
According to a first aspect of the invention there is provided a method of forming a photovoltaic or photoconducting device comprising the laminating together of a first component having a first electrode and a first semiconductive layer predominantly comprising a first semiconductive material, and a second component having a second electrode and a second semiconductive layer predominantly comprising a second semiconductive material, wherein the laminating step involves the controlled joining of said first semiconductive layer and said second semiconductive layer to form a mixed layer comprising proportionally less of said first semiconductive material than said first semiconductive layer and proportionally less of said second semiconductive material than said second semiconductive layer while retaining said first and second semiconductive layers with a reduced thickness.
According to another aspect of the invention there is provided a method of designing and creating a photovoltaic or photoconducting device comprising the steps of: choosing first and second semiconductive materials on the basis of their electronic properties so that said first semiconductive material acts as an electron donor and said second semiconductive material acts as an electron acceptor; forming a first component comprising a first electrode and a first semiconductive layer predominantly comprising said first semiconductive material; forming a second component comprising a second electrode and a second semiconductive layer predominantly comprising said second semiconductive material; and joining the first component to the second component by laminating said first semiconductive layer to said second semiconductive layer. The laminating step may involve the controlled joining of said first semiconductive layer and said second semiconductive layer to form a mixed layer comprising proportionally less of said first semiconductive material than said first semiconductive layer and proportionally less of said second semiconductive layer while retaining said first and second semiconductive layers with a reduced thickness.
Laminating may comprise the application of pressure or heat or pressure and heat. If heat is applied it may involve heating one or both of the semiconductive layers above their glass transition temperatures. The semiconductive layers may be individually treated before lamination, for example by organic or inorganic doping. Such treatment may vary the morphology, the light-absorption characteristics, the transport properties or the injection properties of one or both semiconductive layers. The thickness of the semiconductive layers before lamination may be controlled, for example by spin coating a solution of semiconductive material. Furthermore the thickness of the mixed layer and/or the thickness of the first and second semiconductive layers remaining may be controlled, for example by annealing.
According to a further aspect of the invention there is provided a photovoltaic or photoconducting device comprising: a first electrode; a first semiconductive layer, predominantly comprising a first semiconductive material, over at least part of said first electrode; a mixed layer over said first semiconductive layer; a second semiconductive layer, predominantly comprising a second semiconductive material,over said mixed layer; and a second electrode over at least part of said second semiconductive layer, wherein said mixed layer is connected with the first and second semiconductive layers and has proportionally less of said first semiconductive material than said first semiconductive layer and proportionally less of said second semiconductive material than said second semiconductive layer.
A first substrate may carry or comprise said first electrode and a second substrate may carry or comprise said second electrode. The first and second substrates are preferably self-supporting.
According to a still further aspect of the invention there is provided a photovoltaic or photoconducting device comprising: a first substrate carrying or comprising a first electrode and carrying a first semiconductive layer predominantly comprising a first semiconductive material; a second substrate carrying or comprising a second electrode and carrying a second semiconductive layer predominantly comprising a second semiconductive material; and a third mixed layer, arranged between and connected with the first and second semiconductive layers and having proportionally less of said first semiconductive material than said first semiconductive layer and proportionally less of said second semiconductive material than said second semiconductive layer.
According to any one of the different aspects of the invention, the mixed layer may be an interpenetrating network of the first and second semiconductive materials. The first, and likewise the second, semiconductive materials may comprise a mixture of component materials or a single component material. The first and second substrates and the first and second components may be self-supporting. The semiconductive materials may have the properties as defined in any one of the claims 20 to 36. The first and second semiconductive layers may have the properties, before and after lamination, as defined by claims 38 to 43. The first electrode may make physical contact with the first semiconductive layer, or one of a plurality of layers may be interposed between the first electrode and the first semiconductive layer. Likewise the second electrode may make physical contact with the second semiconductive layer, or one of a plurality of layers may be interposed between the second electrode and the second semiconductive layer. The electrodes may have the same or different work functions. An electrode may itself form one of the self-supporting substrates, or the electrode may be comprised in or carried by one of the self supporting substrates. Preferably one or both of the substrates transmit light. Furthermore, one or both of the substrates (and components) may be flexible.
Lamination techniques are well established, allowing devices to be made in a straightforward way, on a large scale, and at low costs.
The present invention provides for the treatment of the first semiconductive layer and/or second semiconductive layer before effecting lamination. This treatment was not possible in the prior art.
A device made according to the present invention is believed to help avoid the creation of a parallel system of diodes and gives improved performance over the device described in the previously referred to disclosure by Halls et al. The first and second semiconductive layers ensure that a single material does not extend from the first electrode to the second electrode.
A device made according to the present invention has a high efficiency to cost ratio.
Devices made according to the present invention have a high efficiency compared to the previously reported polymer blend device.
Devices made according to the invention make it possible to minimize the risk of forming pin holes or direct conductive paths from one electrode to the other. This is particularly advantageous in the production of large area devices.
As the device is not created in a layer by layer fashion the choice of suitable materials is increased as the effects of creating the second semiconductive layer over the first semiconductive layer in a layer by layer fashion are less critical. This affords greater flexibility in tailoring the device to absorb in specific wavelength regions and allows for a more efficient use of the spectrum when used as a solar cell. It also allows other properties of the device to be controlled or improved, such as its conductivity and serial resistance.
For a better understanding of the present invention and to understand how the same may be put into effect, reference will now be made by way of example only to the enclosed drawings.